Lead tin telluride (Pb.sub.1-x Sn.sub.x Te) is a pseudo-binary system of lead telluride and tin-telluride which forms a solid solution over the entire compositional range wherein 0 .ltoreq. x .ltoreq. 1. The two compounds, lead telluride and tin telluride, are mutually soluble in all proportions and the alloy has an energy gap which varies linearly with composition passing through zero and rising again with increasing tin telluride concentration. This energy gap variability, provided by adjustment of the lead to tin ratio, enables use of this composition for intense radiation sources and intrinsic photodetectors covering the wavelength region from about 5 mm. to the far infrared, for injection laser action to about 28 mm., and for photovolatic detection to 30 mm. in lead tin telluride diodes. As a consequence, lead tin telluride has wide use such as radiation detectors, e.g., in the infrared, laser materials, photosensitive devices, and, in general, semiconductor material.
The phase diagram of lead tin telluride (FIGS. 1 and 2) indicates a narrow separation of the liquidus and solidus curves, thereby enabling the advantageous use of several prior art crystal growth methods. Such methods include the Bridgman-Stockbarger, Czochralski, vapor transport, and vapor phase epitaxial growth techniques. All these methods have been relatively successful in producing lead tin telluride; however, they are deficient in one or more respects.
In the Bridgman-Stockbarger and Czochralski methods, growth proceeds from a melt to a solid. Because the liquidus-solidus curves for lead tin telluride are narrowly separated, the composition of a growing crystal differs from that of the melt from which it grows. Therefore, the resulting crystal does not have a uniform composition but varies, as will be more fully explained with reference to FIGS. 1 and 2.
A further problem arising in the Bridgman-Stockbarger and Czochralski techniques resides in maintaining the proper stoichiometry of the lead and tin metal to tellurium. Because lead tin telluride is stable within a relatively wide solidus field, the ratio of metal to tellurium invariably is not equal to one, but is a number greater or less than one, that is, it can exist having a non-stoichiometric composition. As a consequence, the resulting composition is either metal rich or, most usually, metal poor. Thus, the composition is most often nonstoichiometric.
These prior art techniques further give rise to defects and inhomogeneity as a result of constitutional supercooling. Lead tin telluride exhibits a liquid phase which is increasingly enriched in metal as the liquid phase approaches the liquid-solid interface and further exhibits a sudden drop in metal as the solid phase is entered at the liquid-solid interface. Furthermore, metal-rich portions solidify at a temperature lower than that of tellurium-rich portions. Thus, as the solid forms at the liquid-solid interface, because of the variation in composition between the liquid and solid phases and because of faster solidification of metal-rich portions than tellurium-rich portions due to their lower melting point, the resulting crystal shows metal precipitation and an undesired cellular substructure.
Other problems arise because these techniques require operation at high temperature in order to obtain the melt. Such high temperatures promote a greater likelihood that impurities will be leached, in particular from the crucible, especially in view of the large contact area between the crucible and the crystal. In addition, these methods require relatively elaborate and expensive equipment.
In the vapor transport method, the source, having the desired composition, is place in a temperature gradient for sublimation and condensation on a colder surface. Because growth is initiated by spontaneous nucleation, success depends on the ability to obtain the smallest number of nucleation sites, the control of which is very difficult. Thus, this method usually results in the formation of many small points of nucleation at the tip of the tube and their eventual growth together to produce a crystal which is not a single crystal. Furthermore, these crystals are of a very small size, being limited to a few cubic millimeters, and normally contain a high density of voids or minute holes.
Anneal of lead tin teluride is required to reduce high carrier concentration in the as-grown material. One method includes isothermal metal saturated annealing. The undoped as-grown crystal is sliced into thin wafers and the wafers are isothermally annealed with a metal-rich powder. Due to the slow diffusion rate arising from the low cross-over temperature, the temperature at which the stochiometric line crosses the solidus line, the wafers must be annealed for a long time in the order of two months. Another method is to diffuse lead or other doner impurities into the crystal. The disadvantage with this method, also, is the time factor; it requires approximately two weeks to attain a low carrier concentration.